The present invention relates to the field of thermal printing or recording and, more specifically, to a thermal transfer recording system and method for use in recording a tonal or grey scale image on an ink receiving sheet.
Commonly assigned, copending applications U.S. Ser. Nos. 676,502; 685,714; and 685,715 are directed to closed loop systems and methods for thermally recording a tonal or grey scale image, defined by electronic image signals, on a thermal paper or transparency material which includes an integral thermally sensitive recording layer.
The recorded image is defined by a matrix array of minute pixel areas, each of which has a desired or target density or tone specified by the image signals. Pixel area tone is varied by varying the size of a dot recorded therein in a manner analogous to half-tone lithographic printing.
The nature of the thermally sensitive recording layer is such that dot size progressively increases with increased amounts of thermal energy applied to form the dot. To precisely control dot size, the thermal recording systems disclosed in the above-noted applications employ a closed loop control system in which a dot is optically monitored with a photodetector during formation to determine pixel density. This information is fed back to the control system where it is compared to a signal indicative of target density. Based on this comparison, the control system regulates the application of thermal energy to progressively increase dot size until a predetermined comparison value is achieved. Thereafter, the application of thermal energy is terminated.
The key to achieving precise control over pixel density is to configure the recording system so that the optical monitoring means, i.e. the photodetector, has an unobstructed field of view of dot formation to provide the necessary feed back.
If the recording medium is a thermal paper having an opaque base sheet, thermal energy preferably is applied with a thermal print head from the back side of the paper through the base to form dots in the recording layer on the front side where dot formation may be monitored without obstruction by the print head, as disclosed in the previously mentioned application U.S. Ser. No. 676,502. For transparency materials, the heat is applied with the print head through a light reflective buffer sheet in engagement with the recording layer on the front side, and dot formation is monitored from the back side with a photodetector that looks through a transparent base film to read the reflected light level of the recording layer where a dot is being formed as disclosed in previously mentioned applications U.S. Ser. Nos. 685,714 and 685,715.
In contrast to recording on a thermally sensitive medium that includes an integral thermally sensitive recording layer, another thermal recording method known in the prior art utilizes a thermal transfer ribbon. The ribbon includes a fusible ink or marking layer coated on one side of a flexible base layer or film. The ribbon is placed in contact with an ink receiving sheet, e.g., a plain sheet of paper, with the ink layer in facing relation to the receiving sheet. The base is then selectively heated from the back side. In those areas where the temperature is raised sufficiently to fuse or liquefy the ink, ink transfer occurs to form a mark or dot on the paper.
A major advantage of this type of recording system is that it employs common, inexpensive paper as the receiving sheet and does not require the use of an expensive special purpose thermal paper.
To achieve high quality tonal image recording utilizing thermal transfer techniques, it is essential to precisely control pixel density (dot dize). Therefore, it would be highly desirable to incorporate the dot monitoring and feed back control concept into a thermal transfer image recording system.
Some thermal transfer systems known in the prior art utilize a resistive element print head which heats up in response to a passage of current therethrough. The head is engaged with the back side of the ribbon and applies thermal energy which flows through the base and fuses the ink to effect transfer. Dot formation is not visible for monitoring purposes because it occurs between the opaque receiving paper and the ribbon which also generally is opaque. But, even if dot formation was visible from the back side of the ribbon, the overlying print head would block any opportunity to monitor dot formation with a photodiode for feed back purposes.
Before the feed back control concept can be integrated into a thermal transfer recording system, it will be necessary to solve two problems. First, there must be a visual indication of ink transfer or dot size that is accessible from the back side of the ribbon for monitoring purposes. And secondly, the optical path between the visual indication and the photodetector must not be obscured or blocked by any component that acts on the backside of the ribbon to generate heat therein.
As an alternative to selectively heating a thermal transfer ribbon with an external thermal energy applying device, such as a resistive element print head, some thermal ink ribbons known in the prior art include within their multi-layered structure an electrically resistive layer that serves an internal heating element. In operation, recording signal voltage is applied between a pair of spaced apart electrodes which are in contact with the back side of the ribbon. This causes a current to flow in the resistive layer between the electrode sites. The current flow generates heat in the resistive layer which in turn is transmitted to the ink layer to effect transfer.
For representative examples of resistive layer thermal transfer ribbons, and thermal recording systems and components configured for use therewith, reference may be had to U.S. Pat. Nos. 4,477,198; 4,470,714; 4,458,253; 4,345,845 and 4,329,071. Also see "Thermal Transfer Printer Employing Special Ribbons Heated With Current Pulses", IBM Technical Disclosure Bulletin, Vol. 18, No. 8, January 1976, page 2695.
Above noted U.S. Pat. No. 4,345,845 is directed to a feed back control system for driving the electrodes with a voltage source rather than a constant current driver. The system utilizes as feed back an electrical signal representative of internal ribbon voltage at the print point. However, the disclosure does not contemplate providing a visual indicator that is representative of or proportional to pixel density or dot size.
It is also known to provide an integral resistive layer in an electro-thermal recording sheet for use in facsimile devices. Typically, such a sheet comprises a base or support layer made of paper, a conductive layer, on the base layer, having sufficient resisitvity to produce joule heating in response to current flow therethrough, and a heat sensitive recording layer, which is also somewhat electrically conductive, coated on top of the heat producing conductive layer. Recording signal voltage is applied between spaced electrodes in contact with the top recording layer. The relative resistivity values of the recording and conductive layers are such that current flows from a first electrode through the recording layer to the underlying conductive layer, sideways along the conductive layer towards the second electrode, and then back through the recording layer to the second electrode. The current flow in the conductive layer generates heat which flows upwardly to the recording layer thereabove and causes heat sensitive dyes therein to change color or tone to produce a visible mark or dot.
Representative examples of recording sheets having an internal conductive heating layer overcoated with a conductive and thermally reactive recording layer may be found in U.S. Pat. Nos. 4,133,933; 3,951,757; and 3,905,876 as well as in a paper entitled "Electro-thermo Sensitive Recording Sheets" by W. Shimotsuma et al, Tappi, October 1976. Vol. 59, No. 10, pages 92 and 93.
One advantage of incorporating a resistive heating layer into a thermal transfer ribbon or a thermal recording paper is that the recording signals are applied with spaced apart electrodes which may be configured so that the recorded dot is formed in an area that is aligned with the space between the two electrodes. Because the space is not blocked by a conventional external print head, it has the potential to serve as a "window" for optically monitoring an indicator of dot formation or ink transfer.
Commonly assigned, copending application U.S. Ser. No. 717,122 filed on even date herewith by Irving Erlichman and entitled "Thermal Transfer Recording Medium" discloses and claims a multilayer thermal transfer ribbon. The ribbon comprises a resistive or internal heating element layer in the middle, a fusible ink layer supported on the front side of the resistive layer and a thermally sensitive and electro-conductive indicator layer on the back side of the resistive layer. In response to heat generated in the resistive layer, ink on the front side fuses and transfers to a receiving sheet to form a recorded dot, and a corresponding mark or dot is formed in the indicator layer on the back side. The indicator mark is proportional to the size of the recorded dot or the density of the pixel area in which the dot is recorded, advantageously providing a mark on the back side of the ribbon which may be utilized for feed back control.
Therefore it is an object of the present invention to provide a thermal transfer recording system and method that is configured for recording a tonal or grey scale image on an ink receiving sheet and utilizes a thermal transfer ribbon having a fusible ink layer on one side and an indicator layer on the opposite side which provides an indication of pixel density resulting from transfer of ink from the ink layer to the receiving sheet.
It is another object to provide such a system and method which applies recording signals to the ribbon in a manner that allows the indicator marks to be optically monitored to provide feed back to a control system for accurately controlling recorded dot size or pixel density.
Other objects of the invention will, in part, be obvious and will, in part appear hereinafter.